1. Field of the Invention
The present invention relates to a heavy-duty radial tire, and in particular, to a heavy-duty radial tire which is used for a vehicle which often travels on bad roads.
2. Description of the Related Art
Conventionally, heavy-duty radial tires, and in particular, heavy-duty radial tires used on bad roads, are easily cut when the tires travel over relatively sharp protrusions such as rocks or the like. The cut may extend to the belt layer which is disposed at an inner side in the radial direction of the tire of the tread rubber layer, thereby resulting in so-called cut separation, in which a separation forms between the tread rubber layer and the belt layer, or so-called cut through, in which the cut tears from the outer layer which was cut by the protrusion and penetrates through.
A method of preventing these cuts is known in which, as illustrated in FIG. 6, the angles, which are defined by the equatorial plane of the tire, of respective cords of a first belt layer 74, a second belt layer 76, a third belt layer 78, and a fourth belt tire 80 of a heavy-duty radial tire 70 are made small, e.g., the angles are changed from 25.degree. to 21.degree.. The expansion of the outer diameter of the heavy-duty radial tire 70 is suppressed, rigidity in the circumferential direction is increased, and circumferential direction shearing distortion on the belt is decreased. Further, a method is known of using thick steel cords in the belt layer so as to increase the tearing resistance, which is proportionate to the cord diameter. These methods of uniformly thickening the cord diameters of the entire belt and uniformly decreasing the belt angles defined by the equatorial plane of the tire in order to avoid an excessive concentration of distortion are used in conventional cut resistant structures of belt layers.
However, in conventional heavy-duty radial tires utilizing these types of methods, the methods are effective in dealing with defects caused by gradual tearing from the outer belt layer due to relatively sharp protrusions. However, on rough roads, the vehicle frequently travels over such relatively sharp protrusions.
In such a case, as illustrated in FIG. 7, an entire belt layer 84 of a heavy-duty radial tire 82 is subject to bending deformation in the circumferential direction due to a relatively sharp protrusion 83. As a result, the circumferential direction tensile force T of an inner layer belt 86 at the inner side of the belt layer 84 increases, resulting in the inner belt layer 86 tearing before a belt 88 of the outer layers. The tear extends throughout the entire belt layer 84 at once, leading to a so-called cut burst defect.
Further, as illustrated in FIGS. 8A and 8B, when the total gauge of the belt layer 84 is changed from G1 to G2 due to the use of thick cords, the inner layer tensile force T2 in the circumferential direction increases proportionately to the total gauge in comparison to T1 when the tire is subject to bending deformation in the circumferential direction due to the relatively sharp protrusion 83. Therefore, tearing of the inner belt layer 86 begins at a point P and immediately causes a cut burst defect. Namely, when thick cords are used, the total gauge of the belt layer becomes thicker. Therefore, thick cords are disadvantageous with respect to defects in which the inner layer tears first. Further, reducing the belt angles defined by the equatorial plane of the tire is also disadvantageous for defects in which the circumferential direction tensile force increases and the inner layer tears first due to a sharp protrusion.